A world war is a war involving many of the countries of the world or many of the most powerful and populous ones. World wars span multiple countries on multiple continents, with battles fought in multiple theaters. The term is applied to the two major international conflicts that occurred during the twentieth century: World War I (1914–1918) and World War II (1939–1945) respectively

The Oxford English Dictionary cited the first known usage in the English language to a Scottish newspaper: the People's Journal in 1848, “A war among the great powers is now necessarily a world-war.” The term, “world war” had been used in 1850 by Karl Marx and his associate Friedrich Engels[1] in The Class Struggles in France. Rasmus B. Anderson in 1889 described an episode in Teutonic mythology as a “world war” (Swedish: världskrig), justifying this description by a line in an Old Norse epic poem, "Völuspá: folcvig fyrst i heimi" ("The first great war in the world".)[2] German writer August Wilhelm Otto Niemann had used the term "world war" in the title of his anti-British novel, Der Weltkrieg: Deutsche Träume (The World War: German Dreams) in 1904, published in English as The Coming Conquest of England.

In English, the term "First World War" had been used by Charles à Court Repington as a title for his memoirs (published in 1920) having discussed the matter with a Major Johnstone of Harvard University in September 1918. The term "World War I" was coined by Time magazine in its issue of June 12, 1939. In the same article, the term "World War II" was first used speculatively to describe the upcoming war. The first use for the actual war came in its issue of September 11, 1939.[3] One week earlier, the Danish newspaper Kristeligt Dagblad used the term on its front page, saying "The second World War broke out yesterday at 11 a.m."[4]

Speculative fiction authors had been noting the concept of a Second World War in 1919 and 1920, when Milo Hastings wrote his dystopian novel, City of Endless Night. Other languages have also adopted the "world war" terminology, for example; in French: "world war" is translated as Guerre Mondiale, in German: Weltkrieg (which, prior to the war, had been used in the more abstract meaning of a global conflict), in Italian: Guerra Mondiale, in Spanish and Portuguese: Guerra Mundial, in Danish: Verdenskrig, and in Russian: Мировая война (Mirovaya Voyna.)

In terms of human technological history, the scale of the two "world wars" was enabled by the technological advances of the second industrial revolution and the resulting globalization that allowed global power projection and mass production of military hardware, but wars on such a scale have not been repeated due to the onset of the atomic age and the resulting danger of mutually assured destruction. It had been recognized that the complex system of opposing alliances — the German, Austro-Hungarian, and Ottoman empires against the British, Russian, French empires was likely to lead to a worldwide conflict in the event of war breaking out. Due to this fact, a very minute conflict between two countries had the potential to set off a domino effect of alliances, triggering a world war. The fact that the powers involved had large overseas empires virtually guaranteed that such a war would be worldwide, as the colonies' resources would be a crucial strategic factor. The same strategic considerations also ensured that the combatants would strike at each other's colonies, thus spreading the wars far more widely than those of pre-Colombian times.

The outcome of the world wars had a profound effect on the course of world history. The old European empires had collapsed or been dismantled as a direct result of the wars' crushing costs and in some cases the defeat of imperial powers. The United States of America had been firmly established as the dominant global superpower, along with its ideological foe, the Union of Soviet Socialist Republics, in close competition. These two superpowers exerted political influence over most of the world's nation-states for decades after the end of the Second World War. The modern international security, economic and diplomatic system had been created in the aftermath of the wars.

Institutions such as the United Nations were established to collectivize international affairs, with the explicit goal of preventing another outbreak of general war. The wars had also greatly changed the course of daily life. Technologies developed during wartime had a profound effect on peace-time life as well, for instance; advances in: jet aircraft, penicillin, nuclear energy, and electronic computers.

Since the atomic bombings of Hiroshima and Nagasaki during the Second World War, there has been a widespread and prolonged fear of an upcoming Third World War between nuclear-armed powers. The Third World War is generally considered a hypothetical successor to the Second World War and is often suggested to become a nuclear war, devastating in nature and likely much more violent than the First World War and the Second World War combined. This war has been anticipated and planned for by military and civil authorities, and explored in fiction in many countries. Concepts have ranged from purely conventional scenarios, to limited use of nuclear weapons, to the complete destruction of the planet's surface.

Various former government officials, politicians, authors, and military leaders (including the following people: James Woolsey[7]Alexandre de Marenches,[8]Eliot Cohen,[9] and Subcomandante Marcos[10]) have attempted to apply the labels of the “Third World War” and “Fourth World War” to various past and present military engagements and diplomatic stand-offs since the closing of the Second World War, for example, the Cold War and the War on Terror. Among these are former American, French, and Mexican government officials, military leaders, politicians, and authors: Despite their efforts, none of these wars are commonly deemed world wars.

The Second Congo War (1998–2003) involved nine nations and led to ongoing low-level warfare despite an official peace and the first democratic elections in 2006. It has often been referred to as "Africa's World War".[11] During the early-21st century the Syrian Civil War and Iraqi Civil War and their spillovers worldwide are sometimes described as proxy wars waged between the United States and Russia,[12][13][14][15] which led some commentators to characterize the situation as a "proto-world war" with nearly a dozen countries embroiled in two overlapping conflicts.[16]

The two world wars of the 20th century had caused unprecedented casualties and destruction across the theaters of conflict.[17] There have been several wars that occurred before the 20th century with as many or more casualties than those of the First World War (16,563,868 — 40,000,000), including:

^"Grey Friday: TIME Reports on World War II Beginning". TIME. September 11, 1939. Retrieved 20 October 2014. World War II began last week at 5:20 a. m. (Polish time) Friday, September 1, when a German bombing plane dropped a projectile on Puck, fishing village and air base in the armpit of the Hel Peninsula.

^Stannard, David E. (1993). American Holocaust: The Conquest of the New World. New York & Oxford: Oxford University Press. p. 11. ISBN978-0-19-508557-0. In the 1940s and 1950s conventional wisdom held that the population of the entire hemisphere in 1492 was little more than 8,000,000—with fewer than 1,000,000 people living in the region north of present-day Mexico. Today, few serious students of the subject would put the hemispheric figure at less than 75,000,000 to 100,000,000 (with approximately 8,000,000 to 12,000,000 north of Mexico).